NMR Sample Preparation

There are different types of NMR: Solid state, liquid state, and solution-state NMR.

The way the sample is prepared will depend on the type of NMR. The most common is the solution state. For this, you will require a suitable solvent and the internal standard TMS - Tetramethyl silane.

Equipment: NMR tubes are used as they are designed specifically for the spectrometer. They are typically 5mm in diameter and have a small cap and an NMR adaptor which helps the tube stand upright.

A glass pipette is used instead of a plastic pipette and micropipette because it is much thinner for a more precise transfer of the sample in the thin NMR tube.

How much solid to weigh out: The mass of solid compounds for carbon NMR is typically between 20-50 mg. This is a lot more than the mass required for proton NMR - which is 1-2 mg and is to do with the difference in technique sensitivity.

Solvents: The solvent used is typically deuterated. This means that the hydrogens in the solvent molecules are replaced with the isotope of hydrogen named Deuterium. It helps with the locking and shimming process inside the machine.

Reference Compound: TMS is the chosen reference compound used because the carbons and protons are in the exact same chemical environment and causes a resonance peak at 0 ppm and so lets us know where the chemical shift scale begins.

Adding TMS directly to your sample will be too concentrated which is why you must dissolve a small drop in some solvent before you dissolve your solid.

In the case of a solid sample, these are the steps to follow:

  1. Weigh out 20-50 mg of solid
  2. Add one drop of TMS to some deuterated solvent
  3. Dissolve the solid in solvent mixed with TMS
  4. Pipette the sample solution in the NMR tube
  5. Place the tube in the NMR adaptor
  6. Wipe the tube with tissue paper